1. Field of the Invention
The invention relates to a surface coated cermet cutting tool having a hard coating layer in which an upper layer composed of a property-modified α-type Al2O3 layer that exhibits excellent crystal grain boundary strength. Especially, the invention relates to a surface coated cermet cutting tool (hereafter referred to as a coated cermet tool) which has excellent chipping resistance of the hard coating layer and exhibits excellent wear resistance over a long period of time in a case of high-speed cutting of difficult-to-cut material such as soft-steel, stainless steel, and high-manganese steel that have high viscosity, high adherence to the hard coating layer on the surface of a cutting tool and cause extremely high cutting resistance. Priority is claimed on Japanese Patent Application No. 2005-333539, filed Nov. 18, 2005 the contents of which is incorporated herein with reference.
2. Description of Related Art
In the prior art, a coated hard metal tool having a main body composed of tungsten carbide (WC) based hard metal or titanium carbonitride (TiCN) based cermet, and a hard coating deposited on the surface of the main body (hereafter referred to as a tool body) have generally been known. A configuration of the hard coating is known to have the following lower layer and upper layer.    (a) The lower layer of a Ti compound layer has a 3-20 μm thickness and consists of one or more layers selected from a titanium carbide (hereafter referred to as TiC) layer, a titanium nitride (hereafter referred to as TiN) layer, a titanium carbonitride (hereafter referred to as TiCN) layer, a titanium carboxide (hereafter referred to as TiCO) layer, and a titanium oxycarbonitride (hereafter referred to as TiCNO) layer each of which is formed by chemical vapor deposition.    (b) The upper layer of the hard coating consists of an aluminum oxide layer (hereafter referred to as conventional α type Al2O3 layer) which is formed by chemical vapor deposition so as to have an α type crystal structure and an average thickness of 2 to 20 μm.
It is well known that the above-described conventional coated cermet tool can be employed to the cutting of, for example, various steels of general grades and ordinary cast irons (Japanese Unexamined Patent Application, First Publication No. H6-31503: Patent Reference 1).
It is also known that, in the above-described coated cermet tool, each layer as a constituent of the hard coating layer generally has a granular crystal texture, and in order to enhance the strength of the titanium compound lower layer, TiCN layer as a constituent of the lower layer may be provided with longitudinally growing crystal structure by deposition using a common chemical vapor deposition apparatus, a reaction atmosphere of mixed gas containing organic carbonitride, and a moderate deposition temperature within a range of 700 to 950° C. (Japanese Unexamined Patent Application, First Publication No. H6-8010: Patent Reference 2).
In recent years, cutting machines are given a remarkably high performance. On the other hand, there is a strong demand for labor reduction, energy saving, and cost reduction during cutting operation. In accordance with such circumstances, cutting speed required for the cutting operation tends to be increased.
The above-described conventional coated cermet tools cause no problems when they are used for high-speed cutting of general steel such as low-alloy steel and carbon steel and ordinal cast irons such as gray iron. However, the conventional coated cermet tools reach the end of their tool life within a relatively short time, when these cutting tools are used for high speed cutting of difficult-to-cut materials such as soft steel, stainless steel, and high manganese steel or the like. The short tool life of the conventional coated cermet tool may be explained as follows. The difficult-to-cut material itself has a high viscosity, and during operation, is highly adhesive to the hard coating layer of the surface of the cutting tool. During high speed cutting while generating high-heat, the above-described properties of the difficult-to-cut material are further enhanced and the cutting force is increased. On the other hand, high-temperature strength of the conventional α type Al2O3 layers of the above-described conventional coated cermet tools are not sufficient to resist such a high cutting force. Therefore, cutting edges of the conventional coated cermet tools occur chipping easily during high-speed cutting of difficult-to-cut materials.